1. Field of Invention
The present invention relates to cooling fans. More particularly, the present invention relates to a fan speed pulse filter for a pulse width modulation (PWM) fan.
2. Description of Related Art
Fan cooling is typically used to cool electric system, such as microprocessors and power suppliers, when the power consumption of the system exceeds its natural cooling capacities. However, fan failure can result in overheating, and may cause permanent damage to the system. Therefore, it is desirable to be able to detect a slow fan speed or fan failure and initiate an automatic shutdown to avoid thermal runaway. By equipping a Hall-effect switch or by sensing a fan motor current, many fan systems or fans include a tachometer output, which outputs pulses per revolution of the fan. The tachometer output of the fan is connected to a CPU or input-output (I/O) ports for a fan speed monitor and control.
FIG. 1 illustrates the tachometer output pulses when the fan is operating at full speed. In general, the speed of the fan is measured in revolution per minute (RPM); speed is determined by measuring the period (Tp) of the tachometer pulses. The RPM is defined by RPM=60/(Np*Tp), wherein Np is the pole number of the fan.
Many fan controllers use PWM control, with fan speed determined by the duty cycle of the PWM signal. The PWM mode provides highly efficient fan control and prevents the fan from locking during a start-up period. However, generating the tachometer pulses has the following difficulties.
The tachometer only operates when power is applied to the fan. FIG. 2 shows the output pulses of the tachometer. As shown in FIG. 2, Tp1 is the period of the tachometer pulse, and an incorrect pulse period Tp2 is produced when the PWM signal is off, causing an inaccurate RPM for the fan. However, the tachometer only operates correctly when proper power is applied. When the applied power is incorrect, an inaccurate RPM is output; as a result, the fan is driven by a slow slew of the PWM signal. Namely, noise pulses 20 occur at the rising time Tr or the falling time Tf of the PWM signal, as shown in FIG. 3.
In addition, the tachometer may output phantom pulses when the power is rapidly applied to the fan. FIG. 5 shows the phantom pulses 32 of the tachometer pulse, in which the fan is driven by a fast slew PWM signal.
Noise, the phantom pulse, or an incorrect period of the tachometer pulse introduces an incorrect RPM for the fan. It is necessary to provide a fan speed pulse filter to filter the noise and the phantom pulses and generate a non-interrupted pulse output.